Plugin to add related posts within the contents

krish-related-posts.php

<?php
/*
Plugin Name: Fixed Inline Related Posts
Description: Shows 3 related posts within the content
Version: 1.0.1
Author: krish
*/

//Creating new Custom Fields for posts to add related post title and URL (uses ACF)
if( function_exists('acf_add_local_field_group') ):

acf_add_local_field_group(array(
	'key' => 'group_60daba31d7ca8',
	'title' => 'Inline Related Posts',
	'fields' => array(
		array(
			'key' => 'field_60daba6c5b50a',
			'label' => '(1) Related Post Title',
			'name' => '1_related_post_title',
			'type' => 'text',
			'instructions' => 'Add your link anchor text.',
			'required' => 0,
			'conditional_logic' => 0,
			'wrapper' => array(
				'width' => '',
				'class' => '',
				'id' => '',
			),
			'default_value' => '',
			'placeholder' => '',
			'prepend' => '',
			'append' => '',
			'maxlength' => '',
		),
		array(
			'key' => 'field_60dabb1f5b50b',
			'label' => '(1) Related Post URL',
			'name' => '1_related_post_url',
			'type' => 'url',
			'instructions' => '',
			'required' => 0,
			'conditional_logic' => 0,
			'wrapper' => array(
				'width' => '',
				'class' => '',
				'id' => '',
			),
			'default_value' => '',
			'placeholder' => '',
		),
		array(
			'key' => 'field_60dabb4c5b50c',
			'label' => '(2) Related Post Title',
			'name' => '2_related_post_title',
			'type' => 'text',
			'instructions' => 'Add your link anchor text.',
			'required' => 0,
			'conditional_logic' => 0,
			'wrapper' => array(
				'width' => '',
				'class' => '',
				'id' => '',
			),
			'default_value' => '',
			'placeholder' => '',
			'prepend' => '',
			'append' => '',
			'maxlength' => '',
		),
		array(
			'key' => 'field_60dabb665b50d',
			'label' => '(2) Related Post URL',
			'name' => '2_related_post_url',
			'type' => 'url',
			'instructions' => '',
			'required' => 0,
			'conditional_logic' => 0,
			'wrapper' => array(
				'width' => '',
				'class' => '',
				'id' => '',
			),
			'default_value' => '',
			'placeholder' => '',
		),
		array(
			'key' => 'field_60dabb775b50e',
			'label' => '(3) Related Post Title',
			'name' => '3_related_post_title',
			'type' => 'text',
			'instructions' => 'Add your link anchor text.',
			'required' => 0,
			'conditional_logic' => 0,
			'wrapper' => array(
				'width' => '',
				'class' => '',
				'id' => '',
			),
			'default_value' => '',
			'placeholder' => '',
			'prepend' => '',
			'append' => '',
			'maxlength' => '',
		),
		array(
			'key' => 'field_60dabb905b50f',
			'label' => '(3) Related Post URL',
			'name' => '3_related_post_url',
			'type' => 'url',
			'instructions' => '',
			'required' => 0,
			'conditional_logic' => 0,
			'wrapper' => array(
				'width' => '',
				'class' => '',
				'id' => '',
			),
			'default_value' => '',
			'placeholder' => '',
		),
	),
	'location' => array(
		array(
			array(
				'param' => 'post_type',
				'operator' => '==',
				'value' => 'post',
			),
		),
	),
	'menu_order' => 0,
	'position' => 'normal',
	'style' => 'default',
	'label_placement' => 'left',
	'instruction_placement' => 'label',
	'hide_on_screen' => '',
	'active' => true,
	'description' => '',
	'acfe_display_title' => '',
	'acfe_autosync' => '',
	'acfe_form' => 0,
	'acfe_meta' => '',
	'acfe_note' => '',
));

endif;

//function to add related links in the content
function related_contents($contents) {
    $links = array();
    $found_links = 0;
    for($i=1;$i<=3;$i++) {
        $this_title = get_field($i.'_related_post_title');
        $this_url = get_field($i.'_related_post_url');
        $link[$i] = '<div class="related-post"><a href="'.$this_url.'">'.$this_title.'</a></div>';
        if($this_title != NULL && $this_url != NULL ) {
            $found_links++;
        }
    }
    if($found_links==0) {
        return $contents;
    }

$pattern = "/<p>.*?<\/p>/m"; // Multiline
$paragraph_count = preg_match_all($pattern,$contents);
$para_count = 1;
$para_third = floor($paragraph_count/3)-1;
$paras = explode("</p>", $contents);
$contents = '';
$link_count = 1;
foreach($paras as $para) {
    if(($para_count % $para_third)==0) {
        $contents .= $para.$link[$link_count];
        $link_count++;
    }
    else {
        $contents .= $para;
    }
    $para_count++;
}
$styles = "<style>.related-post{background:#ececec;margin:1.5rem auto;padding:1rem;border-left:5px solid #aeaeae}.related-post a{color:#363636;text-decoration:underline;font-weight:700}</style>";
    return $contents.$styles;
}
add_filter('the_content', 'related_contents');
?>